The xerographic copying process is based upon the fact that certain photoconductive insulators or photoreceptors will accept and hold an electrostatic charge in the dark and will release or dissipate said charge in areas which are exposed to light. Moreover, such elements will release or dissipate their charge at a rate which is proportional to the intensity of the light to which they are exposed, whereby portions of the charged surface which are exposed to light reflected by black or nonreflective imaged areas of an imaged original sheet will retain a strong electrostatic charge, portions exposed to light reflected by grey or semi-reflective imaged areas of an image original sheet will retain a weaker electrostatic charge having an intensity proportional to the degree of reflection and portions exposed to light reflected by white or background will retain little or no electrostatic charge, insufficient to attract or hold toner powder to such portions.
The exposed photoreceptive surface retains latent electrostatic image areas of different intensities having different affinities for oppositely-charged developer powders or liquids. The charged image areas attract different amounts of the developer powder or toner, in proportion to the strength of the charges present, to produce developed image areas which may vary in tone between light grey and dark black to correspond with the imaged areas of the original sheet. Finally the toner images are heatfused to render them permanent, the toner powder containing a heatfusible resinous binder material.
There are many variations of the xerographic copying process. The photoconductive insulating surface may be the surface of a selenium drum, a photoreceptive plastic belt, a photoreceptive coated master web or photoreceptive coated paper sheets. The latent electrostatic images may be developed with toner directly on the photoreceptive surface or the latent images may be transferred to a copy sheet and thereafter developed and heat-fused on the copy sheet. Also, the photoreceptive surface may be present on the final copy sheet so that the latent images are toned and heat-fused thereon to produce the actual copy.
Among the problems encountered with some of the known electrostatic copying processes are the problems of (a) non-uniform photoresponsive sensitivity of the photoconductive insulating surface and (b) interdocument development. The first problem arises from the fact that certain photoconductive insulating surfaces, particularly where such surfaces comprise photoreceptive coatings on paper master webs or sheets, have dielectric or insulating properties which may vary from one master web to the next or from one batch of master sheets to the next. In certain cases the dielectric or insulating properties of the master web or sheet are so great that the photoresponse of the master is too slow, i.e., the photoconductive surface does not release or dissipate the electrostatic charge at a satisfactory rate or speed under the effects of light reflected from an imaged original sheet. In some cases this results in copies having a poor grey scale, i.e., areas which should be lighter than others on the copy sheet are not lighter because the master did not dissipate sufficient electrostatic charge from such areas. In other cases even the background areas of the master retain a sufficiently-strong electrostatic charge that they attract the toner powder to such areas to produce copies having various amounts of developed toner in the form of specks and spots in the non-image background areas.
The second problem, namely interdocument development, occurs in processes which use a charged photoreponsive surface which is longer than the area of said surface which is exposed to the illumnated original sheet. The unexposed areas of the master retain the electrostatic charge and attract toner powder to form transverse black stripes between the copies. This wastes toner powder and produces unsightly black margins at the top and/or bottom of the copies formed, which margins generally are cut away and discarded. Moreover, the heavy deposits of toner formed on the unexposed areas of the photoresponsive surface of the master, and which are not transferred to the copy sheet, must be cleaned from said surface before those areas of the master can be reused. Obviously, such heavy deposits contaminate the cleaning brushes of the copying machine.
The problem of interdocument development can be overcome by limiting the charged areas of the photoreceptive surface to correspond to the length of the original sheets being copied. However, this requires the intermittent operation of the corona discharge element and the repeated activation/deactivation of said device, as occurs for instance when several copies of the same original are being made or when a rapid succession of different copies are being made, can result in a premature breakdown of the corona discharge device resulting from breakage of the charger wires.
The problem of interdocument development can also be overcome by removing the electrostatic charges from the photoreceptive surface in areas between copies by exposing said areas to the light source, i.e., as the continuous charged photoreceptive web is moved into and out of the exposure station, the leading and trailing edges, which are beyond the length of the web actually exposed to the illuminated original sheet being copied, may be exposed to a light source to remove the electrostatic charge from said areas. However, a conventional incandescent or flourescent lamp cannot be turned on and off with sufficient rapidity to remove the electrostatic charge from the interdocument areas of the moving photoresponsive web without interfering with the charge in the areas of the web corresponding to the copies to be made. Thus, when the light is turned off at the leading edge of each intended copy length of the web, the after-glow will remove or reduce the charge near the top of the intended copy length of the web. When said copy length is exposed to an illuminated original sheet having images near the top thereof, said images are not reproduced or are only weakly reproduced on the photoresponsive web since the electrostatic charge has already been removed wholly or in part from said areas of the web. Conversely, when the interdocument area of the web beyond the intended copy area is to be exposed to light source, the slight time lapse between activation of the light source and full illumination results in an incomplete removal of the electrostatic charge from the area of the web just beyond the intended copy area whereby said area remains receptive to the imaging toner and develops a black border at the bottom edge of each copy.
Another problem encountered with xerographic copying machines using magnetic carrier particles for the toner powder is the problem of carry-out of the magnetic carrier particles whereby such particles are attracted to the charged surface of the master and transfer thereto with the toner powder. The cause of this problem is complex but the problem is magnified in cases where the surface potential of the photoresponsive surface of the master is unduly high. The transferred magnetic particles may degrade the quality of the copies produced, may scratch the photoresponsive surface of the master and/or may deposit in the mechanism of the apparatus, causing premature wear.